The proliferation of WiFi networks based on the IEEE 802.11 wireless RLAN (radio local area network) standard, commercially known as WiFi®, Wi-Fi or WIFI, had led to availability of inexpensive WiFi devices and chipsets implementing various aspects of the IEEE 802.11 standards. This makes them attractive for delivering ‘last mile’ voice and data services from a fiber backbone to end users in rural area with low population density, or generally in areas where existing alternatives for the ‘last mile’ service delivery, such as WIMAX, land-line phone networks, TV cable and optical fiber based solutions, are too expensive or unfeasible for other reasons. However, conventional WiFi networks are limited to relatively compact environments, such as homes and small offices, and are not suitable for wide-area networks where spectrum may be re-used in different sectors of a base transceiver station (BTS) site.
The MAC (medium access control) layer of conventional WiFi uses a distributed coordination function (DCF) to coordinate transmission between various WiFi terminals in the network. The DCF of the IEEE 802.11 family of standards typically employ carrier sense multiple access with collision avoidance (CSMA/CA) method for network access, wherein a WiFi terminal wishing to transmit data in a particular channel has to first listen to the channel for a predetermined amount of time to determine whether or not another terminal is transmitting on the channel within the wireless range. If the channel is clear from interference, i.e. “idle,” then the WiFi terminal is permitted to begin the transmission process. If the channel is sensed as “busy,” the WiFi terminal delays its transmission for a random duration of time. Collision avoidance is used to improve CSMA performance by not allowing wireless transmission of a terminal if another terminal is transmitting, thus reducing the probability of collision due to the use of a random truncated binary exponential backoff time.
However, when a WiFi network includes multiple access points (APs) each providing wireless access to a plurality of client terminals, same WiFi channels may be used by different APs in overlapping areas, the conventional WiFi MAC becomes inefficient, and the transmission in all sectors must be synchronized. One approach to building a WLAN network based in WiFi chipsets is disclosed in an article by K. Paul, A. Varghese, et al, entitled “WiFiRe: Rural Area Broadband Access using the WiFi PHY and a Multisector TDD MAC”, published in IEEE Communications Magazine, January 2007, Voi. 45, Issue 1, pp. 111-119, which is incorporated herein by reference. The system described by Paul et al uses WiFi PHY layer of WiFi chipsets and ads a single-channel multisector TDD (time division duplexing) MAC using directional antennas. The WiFiRe MAC of Paul et al is a derivative of the WiMAX MAC which uses the TDD/TDMA approach with ranging in which all uplink terminals are synchronized to the base station and are ordered when to transmit by means of specialized messages such as UL-MAP and DL-MAP transmitted by the base station in a MAC header. One drawback of the WiFiRe system is its insufficient resilience to ISM band interference, as the loss of either of the UL-MAP or the DL-MAP messages would severely affect the uplink and downlink scheduling, causing a multiplicity of errors that may last for a period of time longer than the actual interference event.
An object of the present invention is to address at least some of the deficiencies of the prior art by providing a cognitive-radio type WiFi-based network system and method with efficient interference avoidance.